This invention relates to a printing apparatus and to a method of controlling printing in the printing apparatus. More particularly, the invention relates to an ink-jet printing apparatus for ejecting ink by utilizing thermal energy, and to a method of controlling printing by the printing apparatus.
In a printer of the type which uses an ink-jet printhead to print images by ejecting ink, non-uniformity in the size of the ejected ink droplets leads to a decline in the quality of the printed image, which can also result from an attendant unevenness in density. Therefore, in order to perform high-quality printing, it is desirable that the size of the ink droplets be held constant at all times.
With a printhead of the type which causes ink to foam by heating the ink within the printhead so that the ink is ejected by the pressure produced, droplets of a constant size can be ejected by forming bubbles of a constant size.
If the energy introduced to a heater within the printhead is too low, the jetting of the ink may become unstable. If too much energy is introduced, the heater elements may deteriorate and burn out. In an arrangement where the ink is heated by a heater, therefore, it is vital that the amount of heat produced by the heater be held constant.
A heater board on which heater elements are formed is fabricated through a semiconductor manufacturing process. The circuit that drives these heater elements also is formed on the heater board through the same manufacturing process. The resistance value of a heater element fabricated by semiconductor film-forming techniques varies from one heater board to another depending upon the manufacturing lot. As a consequence, even if the voltage applied to the heater is constant, the heater driving electrical energy introduced to the heater will differ depending upon the resistance value of the heater when the resistance value exhibits the above-mentioned variation. In order for the energy introduced to the heater to be held constant irrespective of this disparity in the resistance value of the heater, it is required that the energy of the heat evolved be adjusted based upon the length of time over which current is passed through the heater.
The specification of Japanese Patent Application Laid-Open No. 10-95116 proposes means which corrects for this variation in heater resistance from one heater board to another. Specifically, a heater board on which a heater element is formed is provided with an element for sensing a variation in the resistance value of the heater, the information acquired from the sensing element is extracted, and a correction is applied by adjusting the driving pulse width, which is a condition of the driving signal applied to the printhead from the printer proper. As a result, the amount of heat evolved by the heater is rendered constant.
To deal with the higher density of driving elements, recent printhead heater boards are fabricated by a CMOS semiconductor manufacturing process in which the process steps are reduced to enable a reduction in cost. In accordance with this process, heaters and MOS transistors are serially connected and the MOS transistors are controlled so as to turn on the desired heaters. In this case, ON resistance, which is the value of resistance when a MOS transistor is turned on, also usually exhibits a variation on the order of several tens of percent.
In a case where a heater board is formed on a semiconductor wafer, the number of chips that can be manufactured from a single wafer can be increased by reducing the area of the heater board, thereby making it possible to raise the yield of manufacture. In terms of lowering cost, therefore, a MOS transistor of small area is preferred.
The ON resistance value of the MOS transistor should be sufficiently small in comparison with the resistance value of the heater in order to suppress the influence of resistance-value variation on energy applied to the heater. Lowering the ON resistance involves enlarging the gate width of the transistor, however, as a result of which the transistor occupies a greater area on the heater board. A transistor having such an area that will reduce the ON resistance value sufficiently is difficult to form on a heater board if it is desired to achieve the reduction in cost mentioned above. If the ON resistance value of a MOS transistor serially connected to a heater exhibits variation, the voltage drop across the MOS transistor will fluctuate and so will the voltage impressed upon the heater. If the resistance value of the heater exhibits variation, then the energy applied to the heater will fluctuate in similar fashion.
The specification of Japanese Patent Application Laid-Open No. 10-95116 proposes a method of sensing a variation in the ON resistance of a MOS transistor. A method of measuring the resistance value of a heater and the ON resistance of a MOS transistor according to this proposal is performed as set forth below.
As shown in the equivalent circuit of FIG. 3, a driving element fabricated with the same design and through the same process as those of a driving element provided on a heater board is disposed as an ON-resistance measurement element on individual heater boards constituting printheads. The ON-resistance measurement element is driven by a signal from a device external to the head, the ON resistance value is calculated from the relationship between applied current and measured voltage, and pulse width, which is a driving condition, is varied using a table that is set up beforehand on the side of the device. If this arrangement is adopted, the energy applied to an electrothermal transducer can be rendered constant from one head to another. If the energy is constant between heads, a uniform printing performance is obtained between heads and printing yield rises. This also eliminates rapid burn-out ascribable to deterioration of the heater element due to application of excessive energy. The end result is enhanced printhead reliability.
The ON resistance value generally is low (approximately 10xcexa9). Accordingly, there are cases where the measurement precision (S/N ratio) of the ON-resistance measurement element, which serves as the driving element whose design is the same as that of the driving element of the electrothermal transducer, is unsatisfactory. In such case, however, it is possible to use an ON-resistance measurement element whose design is altered so as to improve measurement precision. It is required in such case that the relative value of amount of variation be kept unchanged. In the case of an NMOS transistor, this can be dealt with by changing the gate width.
As set forth in the specification of Japanese Patent Application Laid-Open No. 10-95116, it is also possible to adopt an arrangement in which an element for sensing a variation in sheet resistance value is fabricated on a heater board on which an electrothermal transducer is formed, with this element being provided along with the above-mentioned ON-resistance measurement element, as shown in FIG. 4. Here two lines usually are required, namely a signal line from the ON-resistance measurement element and a signal line from the element that senses the variation in sheet resistance value. However, it is preferable to adopt an arrangement in which the signal line from the ON-resistance measurement element and the signal line from the sensing element are connected within the heater board so that a single signal line will suffice. Specifically, an arrangement should be adopted in which the signal line from the ON-resistance measurement element and the signal line from the sensing element are connected in parallel, as illustrated in FIG. 4. By applying a certain signal, e.g., a clock selection signal, to the ON-resistance measurement element, the ON-resistance measurement element, namely the driving element, is turned on and off, whereby the ON resistance of the driving element and the sheet resistance of the electrothermal transducer can be extracted at a single external output terminal.
When the driving element is ON, information (resistance values) from both the ON-resistance measurement element and the element that senses the variation in sheet resistance value can be sensed. When the driving element is OFF, only the information from the element that senses the variation in sheet resistance value can be sensed. If this arrangement is adopted, one signal line lead to the outside will suffice. As a result, without any increase in the cost of the printer per se and printhead, a variation in the printing performance of the printhead can be reduced, yield can be raised and reliably can be enhanced by eliminating early premature burn-out of the electrothermal transducer.
The result of measurement from the ON-resistance measurement element and the result of measuring variation in the sheet resistance value of the heating resistor are thus output to the external terminal via an external output terminal, thereby making it possible to change the driving conditions of the driving element or heating element.
In recent years, however, printheads have come to require the use of a heater board having a long row of nozzles that furnish a greater printing width per scan in order to support high-speed printing. Further, reducing the area of the heater board is important in order to lower cost. The result is a heater board in which the width at right angles to the heater row is comparatively small. The wiring resistance of wiring that supplies power to the heater within the heater board and to the transistor that drives the heater increases owing to the elongated heater board. This is due to the greater distance from a contact pad to the heater or the transistor.
Furthermore, the number of heaters driven simultaneously is greater in order to achieve high-speed printing. In order to avoid a fluctuation in the voltage drop of the wiring, the number of wires within the heater board is increased. If there is no change in the area needed for this wiring, the wiring area per wire will diminish and, hence, wiring resistance per wire will increase.
Since wiring is made of aluminum or the like using semiconductor film-forming techniques, manufacturing variation on the order of several tens of percent usually appears as the resistance value. As a consequence, when wiring resistance rises and becomes so large relative to the heater resistance value as to no longer be negligible, the variation in the resistance value of the wiring connected serially to the heater has a great effect upon the energy introduced to the heater.
Owing to such an increase which cannot be disregarded in the effect of the variation in wiring resistance, it is difficult to calculate the energy introduced to the heater from the resistance value of the heater and measurements of the ON resistance value of the MOS transistor, which drives this heater, by the prior-art circuit set forth in the specification of Japanese Patent Application Laid-Open No. 10-95116.
As a consequence, energy introduced to the heater may be too small or too large owing to a variation in wiring resistance value. This can cause ink to be ejected unstably, resulting in blurred printing. Further, if an excessive amount of energy is applied to a heater, deterioration of the heater hastens and the heater may burn out.
Accordingly, an object of the present invention is to provide a printing apparatus and method of controlling printing that solve the aforementioned problems of the prior art.
According to the present invention, the foregoing object is attained by providing a printing apparatus for performing printing on a printing medium, based upon information transmitted from an external device, by causing a carriage, on which a printhead is mounted, to scan across the printing medium. The printhead comprises: a printing apparatus for performing printing by using the printhead on a printing medium, based upon information transmitted from an external device, the printhead including selection means for selecting one driving element to be driven; a first wiring structure in which the driving element and a monitor resistor element are serially connected; a second wiring structure in which the monitor resistor element and a first power-supply wire are serially connected; a third wiring structure in which the driving element and a second power-supply wire are serially connected; and detecting means for detecting at least one voltage among voltages at a node between the first power-supply wire and the monitor resistor element, a node between the monitor resistor and the driving element, and a node between the driving element and the second power-supply wire. At least one resistance is obtained, based upon the detected voltage, from among resistance of the first and second power-supply wires, resistance of the monitor resistor element and resistance of the driving element, and the printhead is controlled based upon the resistance obtained.
Preferably, the detecting means includes switching means for switching among voltages to be detected, with the switching means switching among detection of voltages at the node between the first power-supply wire and the monitor resistor element, or the node between the monitor resistor element and the driving element, or the node between the driving element and the second power-supply wire, in accordance with a detection control signal.
Preferably, the printhead is an ink-jet printhead for printing by ejecting ink.
Preferably, the printhead is an ink-jet printhead for printing by utilizing thermal energy, the printhead having a thermal energy transducer, which is for generating thermal energy applied to the ink, as the printing element of the printhead.
Further, according to the present invention, the foregoing object is attained by providing a method of controlling printing of a printing apparatus for performing printing on a printing medium, based upon information transmitted from an external device, by causing a carriage, on which a printhead is mounted, to scan across the printing medium. The method includes a selection step of selecting one driving element to be driven; a detection step of detecting, in a first wiring structure in which the driving element and a monitor resistor element are serially connected, a second wiring structure in which the monitor resistor element and a first power-supply wire are serially connected, and a third wiring structure in which the driving element and a second power-supply wire are serially connected, at least one voltage among voltages at a node between the first power-supply wire and the monitor resistor element, a node between the monitor resistor element and the driving element, and a node between the driving element and the second power-supply wire; and a control step of obtaining at least one resistance, based upon the detected voltage, from among resistance of the first and second power-supply wires, resistance of the monitor resistor element and resistance of the driving element. The printhead is controlled based upon the resistance obtained.
Preferably, the detecting step includes a switching step of switching among voltages to be detected, the switching step switching among detection of voltages at the node between the first power-supply wire and the monitor resistor element, or the node between the monitor resistor element and the driving element, or the node between the driving element and the second power-supply wire, in accordance with a detection control signal.
Further, according to the present invention, the foregoing object is attained by providing a printing apparatus for performing printing on a printing medium, based upon information transmitted from an external device, by causing a carriage, on which a printhead is mounted, to scan across the printing medium. The printhead includes any one of voltage detecting means for detecting at least one voltage among voltages at a node between a first power-supply wire and a monitor resistor element, a node between the monitor resistor element and a driving element, and a node between the driving element and a second power-supply wire; and any one of current application means for applying current to at least one of the nodes; wherein at least one resistance is obtained, based upon applied current or detected voltage, from among resistance of the first and second power-supply wires, resistance of the monitor resistor element and resistance of the driving element, and the printhead is controlled based upon the resistance obtained.
Preferably, the pressure detecting means includes switching means for switching among voltages to be detected, with the switching means switching among detection of voltages at the node between the first power-supply wire and the monitor resistor element, or the node between the monitor resistor element and the driving element, or the node between the driving element and the second power-supply wire, in accordance with a detection control signal.
Preferably, the current application means includes current switching means for switching over application of current, with the current switching means switching between detection of voltages at and application of current to the node between the first power-supply wire and the monitor resistor element, or the node between the monitor resistor element and the driving element, or the node between the driving element and the second power-supply wire, in accordance with a control signal.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.